The present invention relates to an improved method and apparatus for separating chrominance and luminance information from a quadrature modulated video color signal. More specifically, the method and apparatus applies adaptive comb filtering techniques, as well as vertical transition detection, to a quadrature modulated video color signal to provide improved separation of chrominance and luminance signal components during aperiodic transition states.
The NTSC color television signal includes a brightness or luminance frequency (Y) signal ranging in frequency from direct current to a nominal bandwidth of 4.2 MHz, and a 3.58 MHz subcarrier which is modulated in phase and amplitude to represent hue and saturation of the image. Typically, the subcarrier is demodulated to produce color difference signals R-Y, B-Y, and G-Y, which are combined with the Y signal for reproduction of red, blue, and green signal information. As is well known, simple filters have long been used to separate the chrominance from the luminance in television receivers. In particular, the composite video is typically fed to a low pass filter to provide the luminance or luma, and to a band pass filter to provide the chrominance or chroma.
In addition to other disadvantages, simple filters have had problems in providing complete separation into chroma and luma components. For example, in order to separate the subcarrier signal for adjusting its amplitude for color intensity control or for demodulation, it is desirable to select only the subcarrier and its side bands without also including high frequency brightness components. Conversely, the full range brightness or Y signal will include the color subcarrier, even if the Y range is limited since the subcarrier modulation may extend below 3.58 MHz by over 1 MHz. Simply stated, there is some high frequency luma or cross chroma that appears in the spectrum of the chroma, and there is some high frequency chroma or cross luma that appears in the spectrum of the luma. Therefore, simple filters can not completely separate a composite video into chroma and luma. The appearance of either the Y or luma signal in the subcarrier or the subcarrier in the Y signal introduces undesirable patterns and distortion of the reproduced television image.
Comb filters have been used to separate interleaved components in a complex spectrum with minimum degradation, and therefore have certain advantages over the use of simple filters (i.e. low/high frequency or band reject filters). For example, U.S. Pat. No. 3,542,945 to N. W. Parker describes a comb filter wherein a composite video signal is subtracted from a second input that has been delayed by one horizontal scan. Since two successive lines of NTSC chroma subcarrier are 180.degree. out of phase by reference to horizontal synchronization pulses, the chroma inputs combine as a subcarrier sum. Since lines of luminance are originally in phase, the combination of the opposite phase luminance component inputs to the subtractor result in cancellation of luminance. Thus, a comb filtered chrominance output was provided in which luminance components had been phase cancelled. Parker's circuit then subtracted the separated chrominance without further phase reversal from the composite video input signal (luminance +chrominance). Phase cancellation of chrominance then occurred which provided a separated luminance output.
Parker's combed filter technique worked well in static pictures having low chrominance levels. However, sudden color changes along a vertical axis and other dynamic changes in chrominance signals within the television picture caused highly visible aberrations and artifacts that caused deviation from the true picture. U.S. Pat. No. 4,240,105 to Faroudja describes the need to switch off the comb filtering during chroma or luma vertical transitions. In particular, Faroudja describes a combed filter that selectively separates chrominance and luminance components from a composite video signal to provide a combed chrominance output and an adaptively combed luminance output. As described, a low frequency luminance vertical transition detector is connected to an input and detects vertical amplitude transitions occurring in adjacent horizontal scanning lines to produce a first switching signal. A second transition detector detects the simultaneous occurrence of transitions in chrominance and high frequency luminance in either direction above a predetermined threshold value to provide a second switching signal. A subcarrier frequency band reject filter in the luminance path, connected to be responsive to one or both of the first and second switching signals, selects subcarrier frequency band reject luminance in lieu of combed luminance during an interval corresponding one or both of the switching signals. The system further includes a switch connected to switch to chrominance bandpass filtered video in lieu of combed chrominance during an interval of one or both of the first or second switching signals. One drawback of the above described arrangement was that the chroma transition detector relied on comparing differences in the amplitudes or magnitudes. This permitted chroma signals which were equal in magnitude but opposite in phase to fool or remain undetected by the chroma transition detector. For example, a vertical transition between cyan and magenta would go undetected.
Another disadvantage is that the simple filter separation of the chroma and luma are along two different paths. Therefore, if they are encoded or recombined for subsequent use, the resulting composite video is not an accurate reproduction of the original composite video; some frequency components have been discarded.
U.S. Pat. No. 4,050,084 to Rossi describes a typical three tap comb filter which is utilized in NTSC color televisions. As described, a typical comb filter utilizes three adjacent television lines in a given field and selectively adds and subtracts them to obtain the chrominance and luminance signals. If three sequential television lines are labeled "top" (T), "middle" (M), and "bottom" (B), a comb filter using one of the following algorithms may be used to separate chrominance (C) and luminance (Y) within the chrominance pass band. EQU C=1/2[M+1/2(T+B)] Equation No. 1 EQU C=1/2[M+T] Equation No. 2 EQU C=1/2[M+B] Equation No. 3
An examination of the function of these algorithms reveals that the comb filter effectively operates by sampling and averaging, with particular weighting coefficients, three picture elements from three adjacent lines in the case of algorithm (1), and two picture elements from two adjacent lines in the case of algorithms (2) and (3). Algorithm (1) is desirably used whenever the chrominance or the luminance on the television lines being combed are reasonably constant. However, if the television picture should contain a luminance or a chrominance transition in the vertical direction, algorithm (1) will generate a disturbing transient in the reconstructed television picture. Accordingly, there is a need for an automatic adaptive comb filter capable of detecting vertical transitions and, in response thereto, to utilize algorithm (2) or (3), or to bypass the comb filter, to minimize the transient distortion.